The present invention relates generally to intra-aortic balloon pump therapy, and more particularly, to systems for inflating and deflating intra-aortic balloons. Still more particularly, the present invention relates to methods for use in setting the timing of the balloon inflation and deflation cycles in intra-aortic balloon-pump therapy.
Intra-aortic balloon pump (IABP) therapy is a form of temporary cardiac assist which is frequently prescribed for patients who have suffered a heart attack or some other form of heart failure. In such therapy, a thin balloon is inserted, typically through the femoral artery, into the patient's descending aorta. The balloon is connected through a series of thin tubes to a control apparatus which causes the balloon to inflate and deflate in time with the patient's heartbeat. The balloon therapy supports the left ventricle of the heart by increasing perfusion of the coronary arteries and by reducing left ventricular work. Coronary artery perfusion is increased by augmenting aortic pressure during the diastolic phase of the cardiac cycle. Left ventricular work is reduced by reducing aortic pressure at the end of diastole, i.e., at the onset of ventricular ejection.
The inflation/deflation apparatus supplies positive pressure for expanding the balloon during an inflation cycle and negative pressure for contracting the balloon during a deflation cycle. In a conventional apparatus, such as that shown schematically in FIG. 1, an intra-aortic balloon 10 is surgically inserted into a patient's descending aorta and is connected through a thin catheter 12 and a large diameter extender 14 to an isolator 18 divided by a pliant membrane 20 into a primary side 22 and a secondary side 24. The entire volume between membrane 20 and balloon 10 is filled with a shuttle gas, such as helium, supplied by a gas source 26. A positive pressure source 28 is connected through a solenoid valve 30 to the input or primary side 22 of isolator 18. Similarly, a negative pressure source 32 is connected through a solenoid valve 34 to the input or primary side 22 of isolator 18. The primary side 22 of isolator 18 is also connected through a solenoid valve 36 to a vent or exhaust port 38.
A central lumen 40 extends from an open end 42 protruding from the tip of balloon 10, through the balloon, catheter 12 and a Y junction 44, to a pressure transducer 46. A fluid, such as saline, fills lumen 40 so as to establish a continuous fluid column therein. As a result, the blood pressure in the descending aorta is transmitted by hydraulic coupling through the fluid column to pressure transducer 46 where it can be measured.
During an inflation cycle, solenoid valve 30 is opened to permit positive pressure from positive pressure source 28 to enter the primary side 22 of isolator 18. This positive pressure causes membrane 20 to move toward secondary side 24, thereby forcing the helium in the secondary side to travel toward and inflate balloon 10. For deflation, solenoid valve 30 is closed and solenoid valve 36 is opened briefly to vent the gas from primary side 22, after which valve 36 is closed. Solenoid valve 34 is then opened, whereupon negative pressure source 32 creates a negative pressure on the primary side 22 of isolator 18. This negative pressure pulls membrane 20 toward primary side 22, whereby the helium is drawn out from the balloon.
In order to achieve optimal therapeutic benefits from the use of an IABP system, the inflation and deflation cycles must be properly synchronized to the cardiac cycle. In particular, the balloon 10 must be inflated and deflated within the diastolic interval, when the left ventricle of the heart is inactive. The diastolic interval is determined based on the pressure waveform in the descending aorta, as transmitted from the tip of the balloon through lumen 40 to pressure transducer 46. The dicrotic notch on the waveform indicates the start of the diastolic interval (i.e., when the aortic valve has closed and left ventricular flow has ended), and the onset of systole defines the end of the diastolic interval (i.e., the beginning of left ventricular flow).
The effect of the IABP on aortic pressure is illustrated in the series of graphs shown in FIG. 2. For purposes of illustration, the figure shows time-aligned “assisted” and “unassisted” aortic pressure waveforms. The assisted waveform shows correct timing of balloon inflation and deflation with respect to the cardiac cycle (as represented by the electrocardiogram (ECG) waveform). Also shown in appropriate time alignment is the balloon volume reflecting the deflated and inflated conditions. Examples of early, late and correct timing for both inflation cycles and deflation cycles are shown in the graphs of FIG. 3.
To maintain correct timing on a continuous basis, the timing of the inflation and deflation cycles must be adjusted to accommodate changes in the patient's heart rate, rhythm and left ventricular contractility. In early IABP systems, this was accomplished manually through continuous operator monitoring of the IABP's aortic pressure waveform and adjustment of separate inflation and deflation timing controls based on certain cardiac events or landmarks. In current IABP systems, the maintenance of proper timing has been made semi-automatic. Inflate timing is initially set automatically based on the Q-S2 interval determined from the previous R-R interval, and is adjusted manually by the operator based on the position of the augmentation curve relative to the dicrotic notch on the aortic pressure waveform. Thereafter, the proper inflate timing is estimated through the use of predefined regression algorithms. These algorithms adjust timing continuously, on a beat-to-beat basis, based on the R-R interval of the prior beat. Because changes in the patient's clinical status or medications can affect the contractility of the left ventricle, coronary artery blood flow, end diastolic volume and pressure, and heart rate, occasional operator intervention is needed to adapt the timing to these clinical changes or to audit and correct the estimation process. To provide a visual reference while setting timing, operators often set the IABP to assist (i.e., to inflate) every other heartbeat, rather than every heartbeat. By assisting every other heartbeat, operators are more easily able to visually detect timing errors by comparing assisted and unassisted pressure waveforms. Furthermore, the aortic pressure waveform during unassisted beats is free of artifacts that can be induced by balloon inflation, catheter movement, etc., thereby making the landmarks for proper inflation and deflation timing clearly visible.
Despite the improvements in operation achieved through partial automation of the inflation and deflation timing, IABP therapy still requires frequent adjustment and monitoring by an operator, and is subject to operator error both at startup and during periodic adjustment. It therefore would be desirable to fully automate IABP timing so as to eliminate the need for constant monitoring by an operator and avoid the potential for such operator errors.
Any attempt to fully automate IABP timing must take into consideration the requirement that IABP therapy be performed during the diastolic interval. More particularly, the balloon inflation cycle is preferably initiated so that augmentation of the aortic pressure begins at the start of the diastolic interval, i.e., at the dicrotic notch. Similarly, the balloon deflation cycle is preferably initiated so that full deflation is reached at the end of the diastolic interval, i.e., at the beginning of systole. If the commands to inflate or deflate the balloon are issued at the time these cardiac events are detected, the resultant timing would be persistently late. This is a consequence of the delays that are inherent in IABP systems. For example, if the balloon inflation cycle is initiated at the time the dicrotic notch is detected, the intrinsic time delays in the IABP system would result in a late augmentation of the aortic blood pressure. Similarly, if the balloon deflation cycle is initiated at the time the end of the diastolic interval is detected, the intrinsic time delays would cause complete deflation of the balloon to occur too late. Although the use of earlier surrogates to predict the occurrence of these cardiac events has been considered, such surrogates have generally proven to be unreliable.
The time delays intrinsic to IABP systems consist of several components. One source of delay is the electro-pneumatic delay associated with the activation of valves, the pressurization of pneumatic volumes and the movement of the shuttle gas to begin inflation or deflation of the balloon, all of which follow a balloon inflation or deflation command. Another source of delay is the time for the shuttle gas to move substantially into (i.e., inflate) or out from (i.e., deflate) the balloon. Yet a further delay is the time from the closing of the aortic valve until the aortic pressure change resulting from that event (i.e., the dicrotic notch) is propagated to the blood pressure monitoring site. For central lumen monitoring, such as depicted in FIG. 1, the monitoring site is the tip of the balloon 10. Still another source of delay is the time it takes the pressure signal to propagate from the monitoring site until its conversion into an electrical signal. For central lumen monitoring, this is the time for the pressure signal to propagate from the tip of balloon 10 to pressure transducer 46.
These pressure delays can best be understood by reference to FIG. 4. This figure shows an ECG waveform followed by a waveform indicating the blood pressure as it exists in the descending aorta. Because of the delays caused by the propagation of blood pressure changes from the aortic valve to the pressure monitoring site and from the monitoring site until its conversion into an electrical signal, the blood pressure waveform as displayed on the console of an IABP system will lag in time from the blood pressure in the descending aorta. This lag in time can be seen in the perceived time shift in the third waveform shown in FIG. 4 which is the blood pressure as measured by transducer 46.
The electro-pneumatic delay is dominated by the contribution of the solenoid activated pneumatic valve which opens a pathway between the balloon and a pressure source during inflation, and between the balloon and a vacuum source during deflation. The excursion of this valve is highly repeatable, such that the delay associated with the valve movement is repeatable from patient to patient and cycle to cycle of the IABP. The time to transfer the shuttle gas into and out from the balloon also varies little from cycle to cycle. Accordingly, these two delay criteria may be considered constants.
The delays associated with propagating changes in aortic pressure from the aortic valve to the blood pressure monitoring site and with propagating pressure signals from the monitoring site until they are converted into electrical signals, however, are highly variable. Factors which impact these pressure delays include the location of the balloon, the location of the blood pressure monitoring site, the quality of the hydraulic coupling, and electronic monitoring considerations. There also may be other variable or constant intrinsic delays in an IABP system. For example, while the electro-pneumatic delay and the shuttle gas transfer time may be considered constants, their values are only estimates based on collected data. Errors in these estimates may result in actual delays that are greater than or less than the estimated values. The total amount of intrinsic delay in an IABP system regardless of its source, less any estimated constants ascribed to specified events, such as the electro-pneumatic delay and shuttle gas transfer time, are referred to collectively herein as the arterial pressure delay (APD).
When an operator adjusts IABP inflation and/or deflation timing based on differences between the appearance of the augmentation curve on the pressure waveform and the appearance of the dicrotic notch, what he is implicitly doing is adjusting the timing to account for the APD. That is, the operator is making the timing sooner or later to account for intrinsic delays in the IABP system so that the inflation and deflation commands will be issued at the appropriate times to enable the effects of inflation and deflation to be realized at the proper time relative to cardiac events.
As the optimal timing of IABP therapy requires a knowledge of the cumulative intrinsic delays, and as the highly repeatable delays are known, there exists a need for a method for automatically determining the APD for each patient. Preferably, such method will enable the APD to be determined accurately and quickly so that the IABP timing can be optimally set not only on a patient by patient basis at the initiation of therapy, but also at periodic intervals during therapy to assure that the timing remains optimal despite changes in the patient's cardiac performance.